Embodiments of the present invention include copolymers of ethylene and propylene, in the substantial absence of dienes. More specifically, the copolymers are made in a process that employs a single reactor, in steady state.
Ethylene propylene copolymers made with metallocene catalysts are known. Many such copolymers are intermolecularly heterogeneous in terms of tacticity, composition (weight percent comonomers) or both. Further, such polymers may also, or in the alternative, be compositionally heterogeneous within a polymer chain. Such characteristics may be, but are not always, the result of multiple reactor schemes or sequential addition of polymer.
The elasticity, flexural modulus and tensile strength of such copolymers, when considered in the aggregate, may not reach a satisfactory level for use in commercial elastomeric operation.
U.S. Pat. No. 5,747,621 suggests fractionable reactor blend polypropylenes, directly obtainable from the polymerization reaction of propylene having 30 to 90% by weight of a boiling n-heptane fraction, soluble in xylene at 135xc2x0 C. In Table 2 of this document, the only fractionation disclosed, each of the solvents appears to be at its boiling point. Further, reference to this table shows that the diethyl-ether fraction has no melting point (amorphous).
In the journal articles Science, Vol. 267, pp 217-219 (1995); Macromolecules, Vol. 31, pp 6908-6916 (1998); and Macromolecules, Vol. 32, pp 8283-8290, pp 3334-3340 and pp 8100-8106, propylene polymers with similar characteristics as those disclosed in the above discussed U.S. Pat. No. 5,747,621 are made and fractionated. The polymers are made with bis(aryl indenyl) or bisindenyl metallocene catalysts. In these journal articles, these polymers are fractionated in boiling ether and heptane, leaving a portion of the polymer insoluble in either. The polypropylenes are stated to be compositionally heterogeneous in terms of tacticity and molecular weight.
U.S. Pat. No. 5,504,172 suggests a propylene elastomer that has properties such that:
(a) the elastomer contains propylene units in an amount of 50 to 95% by mol and ethylene units in an amount of 5 to 50% by mol;
(b) a triad tacticity of three propylene units-chains consisting of head-to-tail bonds, as measured by 13C NMR, is not less than 90.0%; and
(c) a proportion of inversely inserted propylene units based on the 2,1-insertion of a propylene monomer in all propylene insertions, as measured by 13C NMR, is not less than 0.5%, and a proportion of inversely inserted propylene units based on the 1,3-insertion of a propylene monomer, as measured by 13C NMR, is not more than 0.05%.
U.S. Pat. No. 5,391,629 suggests block and tapered copolymers of ethylene with an xcex1-olefin. The copolymers are made by a process of sequentially contacting ethylene with an xcex1-olefin monomer in the presence of an activated cyclopentadienyl catalyst system.
EP 0 374 695 suggests ethylene-propylene copolymers and a process for preparing them. The copolymers have a reactivity ratio product, r1r2, between 0.5 and 1.5 and an isotactic index greater than 0 percent. The copolymers are produced in the presence of a homogeneous chiral catalyst and an alumoxane co-catalyst.
There is a commercial need therefore for an ethylene propylene copolymer that will show a melting point and an excellent balance of elasticity, flexural modulus and tensile strength.
We have discovered that ethylene-propylene copolymers, when produced in the presence of a metallocene catalyst and an activator, in a single steady state reactor, show a surprising and unexpected balance of flexural modulus, tensile strength and elasticity. Moreover, these and other properties of the copolymers show surprising differences relative to conventional polymer blends, such as blends of isotactic polypropylene and ethylene-propylene copolymers.
In one embodiment, the copolymer includes from a lower limit of 5% or 6% or 8% or 10% by weight to an upper limit of 20% or 25% by weight ethylene-derived units, and from a lower limit of 75% or 80% by weight to an upper limit of 95% or 94% or 92% or 90% by weight propylene-derived units, the percentages by weight based on the total weight of propylene- and ethylene-derived units. The copolymer is substantially free of diene-derived units.
In various embodiments, features of the copolymers include some or all of the following characteristics, where ranges from any recited upper limit to any recited lower limit are contemplated:
(i) a melting point ranging from an upper limit of less than 110xc2x0 C., or less than 90xc2x0 C., or less than 80xc2x0 C., or less than 70xc2x0 C., to a lower limit of greater than 25xc2x0 C., or greater than 35xc2x0 C., or greater than 40xc2x0 C., or greater than 45xc2x0 C.;
(ii) a relationship of elasticity to 500% tensile modulus such that
Elasticityxe2x89xa60.935M+12,
or
Elasticityxe2x89xa60.935M+6,
or
Elasticityxe2x89xa60.935M,
where elasticity is in percent and M is the 500% tensile modulus in megapascal (MPa);
(iii) a relationship of flexural modulus to 500% tensile modulus such that
Flexural Modulusxe2x89xa64.2e0.27M+50,
or
Flexural Modulusxe2x89xa64.2e0.27M+30,
or
Flexural Modulusxe2x89xa64.2e0.27M+10,
or
Flexural Modulusxe2x89xa64.2e0.27M+2,
where flexural modulus is in MPa and M is the 500% tensile modulus in MPa;
(iv) a heat of fusion ranging from a lower limit of greater than 1.0 joule per gram (J/g), or greater than 1.5 J/g, or greater than 4.0 J/g, or greater than 6.0 J/g, or greater than 7.0 J/g, to an upper limit of less than 125 J/g, or less than 100 J/g, or less than 75 J/g, or less than 60 J/g, or less than 50 J/g, or less than 40 J/g, or less than 30 J/g;.
(v) a triad tacticity as determined by carbon-13 nuclear magnetic resonance (13C NMR) of greater than 75%, or greater than 80%, or greater than 85%, or greater than 90%;
(vi) a tacticity index m/r ranging from a lower limit of 4 or 6 to an upper limit of 8 or 10 or 12;
(vii) a proportion of inversely inserted propylene units based on 2,1 insertion of propylene monomer in all propylene insertions, as measured by 13C NMR, of greater than 0.5% or greater than 0.6%;
(viii) a proportion of inversely inserted propylene units based on 1,3 insertion of propylene monomer in all propylene insertions, as measured by 13C NMR, of greater than 0.05%, or greater than 0.06%, or greater than 0.07%, or greater than 0.08%, or greater than 0.085%;
(ix) an intermolecular tacticity such that at least X % by weight of the copolymer is soluble in two adjacent temperature fractions of a thermal fractionation carried out in hexane in 8xc2x0 C. increments, where X is 75, or 80, or 85, or 90, or 95, or 97, or 99;
(x) a reactivity ratio product r1r2 of less than 1.5, or less than 1.3, or less than 1.0, or less than 0.8;
(xi) a molecular weight distribution Mw/Mn ranging from a lower limit of 1.5 or 1.8 to an upper limit of 40 or 20 or 10 or 5 or 3;
(xii) a molecular weight of from 15,000-5,000,000;
(xiii) a solid state proton nuclear magnetic resonance (1H NMR) relaxation time of less than 18 milliseconds (ms), or less than 16 ms, or less than 14 ms, or less than 12 ms, or less than 10 ms;
(xiv) an elasticity as defined herein of less than 30%, or less than 20%, or less than 10%, or less than 8%, or less than 5%; and
(xv) a 500% tensile modulus of greater than 0.5 MPa, or greater than 0.8 MPa, or greater than 1.0 MPa, or greater than 2.0 MPa.
The copolymer be made in the presence of a bridged metallocene catalyst, in a single steady-state reactor. Thus, in another aspect, the present invention is directed to a process for producing an ethylene-propylene copolymer having some or all of the above-recited characteristics, by reacting ethylene and propylene in a steady-state reactor under reactive conditions and in the presence of a bridged metallocene catalyst.